In recent years, an organic EL element that spontaneously emits light at a driving voltage as low as about several volts has been attracting attention. The organic EL element has excellent features such as surface emitting characteristics, a light weight, and visibility. Accordingly, the element has been used as a thin display, lighting equipment, or a head-mounted display in order that those excellent features may be exploited.
By the way, an organic EL element constituting a pixel in an organic EL display apparatus is of a structure in which a pair of electrodes and a functionally separated organic compound layer each having a thickness of several tens of nanometers to several hundreds of nanometers are laminated. In addition, the thickness of each layer constituting the organic EL element is comparable to the wavelength of light and hence the organic EL element has the following property. Its emission efficiency largely changes owing to an influence of optical interference depending on the construction of the element. It is to be noted that an improvement in emission efficiency can reduce the power consumption of an apparatus using the organic EL element (such as a display).
As described above, the emission efficiency of the organic EL element is strongly affected by the optical interference, and hence the emission efficiency of the organic EL element largely fluctuates depending on the refractive index and thickness of the organic compound layer. However, no specific technology or approach for the optimization of the emission efficiency of the organic EL element through the utilization of the thickness and refractive index of the organic compound layer provided between the pair of electrodes has been established. However, the behavior of light in an organic EL element can be calculated by an optical simulation introduced in, for example, Non Patent Literature 1. Calculation methods for the reflectance, transmittance, phase shift, and the like of an optical multilayer thin film have also been known.
Meanwhile, an approach to introducing a low-refractive index layer between an electrode and an emission layer has been disclosed. Patent Literature 1 proposes, for example, the following approach for the purpose of improving the emission efficiency of an organic EL element. An organic compound layer is placed between a hole injection layer and a hole transport layer, the organic compound layer having a refractive index smaller than that of each adjacent layer. In addition, Patent Literature 2 discloses that when a hole transport layer constituting a blue-light-emitting element is of a construction formed of two layers having different refractive indices, the emission efficiency of the element changes depending on a thickness ratio between the two hole transport layers.